1. Field of the Invention
The present invention relates generally to automatic welding systems and, more particularly, to an automatic welding system for precisely welding end caps to nuclear fuel rods.
2. Description of the Prior Art
Welding end caps onto a nuclear fuel rod is a critical operation since failure of the weld can result in a leaking fuel rod that may result in an unscheduled plant outage. Such outages are extremely expensive and may result in an unnecessary increase in radiation exposure to plant personnel. While vision systems have been tried to help improve weld quality, such systems have had significant difficulties in viewing highly reflective surfaces such as zirconia fuel rods.
U.S. Pat. No. 5,359,672 to Okumura et al., discloses a method of welding using parallel windows of an image to align welds. Prior to welding, data from the windows is used to recognize a weld line, and the inclination and characteristic values of the weld line. Each of the rectangular windows are deformed into a parallelogram-shaped window along the inclination of the detected weld line.
U.S. Pat. No. 5,554,837 to Goodwater et al., discloses an interactive laser welder having a vision system, which digitizes the weld area to determine the weld path.
U.S. Pat. No. 5,572,102 to Goodfellow et al., discloses a vision system for controlling welding robots. The vision system includes an unoriented light source for illuminating a part to be welded so that a light intensity gradient is created along a seam to be welded. An image of the light intensity gradient is captured by one or more solid-state video cameras. The image is compared with a stored image of a reference so that a deviation of the seam to be welded with respect to the seam of the reference part can be computed for each of a plurality of predefined reference points. The deviation at each reference point is translated by a microprocessor into a coordinate adjustment which is downloaded to a controller for the robot. The controller converts the learned coordinates of the reference part into the actual coordinates of the seam to be welded using the coordinate adjustments provided by the microprocessor, permitting the robot to weld the seam without vision sensing or weld path adjustments during the welding process.
U.S. Pat. No. 5,511,007 to Nihei et al., provides a method in which data obtained from a real-time sensor is stored during the operation of a robot, such as a welding robot, for later diagnosis of the operational state of the sensor. The method includes the steps of scanning an object by using a laser sensor to obtain sectional data, detecting a weld line position on the basis of the sectional data, storing in memory the weld line position along with the robot positions corresponding to the time of detection, and storing in memory the sectional data when a predetermined robot position is reached. The operational state of this sensor is traced on the basis of the weld line position and the sectional data stored in memory. Various parameters for detection are set so that an optimum state for detection is established.
None of the above-referenced patents appear to solve the problem of viewing a highly reflective fuel rod or to recognize the importance of measuring the centerline of the electrode with respect to the centerline of the pre-weld arc gap to improve the quality of the weld. In addition, none of the references appear to use this information before and after the welding operation to provide an indication of weld quality.
Thus, there remains a need for a new and improved welding system which is sufficiently sensitive to detect an arc gap of a highly reflective metal rod while, at the same time, uses this information to accept or reject the quality of the end cap weld.